1. Field of the Invention
The present invention relates to an optical pickup device for focusing laser light on a recording surface of an optical disk to perform recording and reproduction of information and, more specifically, to a technology effective to be used in an optical disk device which supports a high-capacity optical disk.
2. Description of the Related Art
In an optical pickup device in the related art, as shown in JP-A-2002-123969, a focus error signal from reflected light of an optical disk is generated using an astigmatic method, and an objective lens is moved in a focusing direction based on the focus error signal to perform focusing control. In the same manner, tracking control is performed by dividing laser light into three beams by a diffraction grating, and generating a tracking error signal based on a three spot method and a differential push-pull (DPP) method from three-beam reflected light, or generating a tracking error signal based on a phase difference detecting (DPD) method from single-beam reflected light, and then moving the objective lens in a tracking direction based on these tracking error signals.
However, in the optical pickup device disclosed in JP-A-2002-123969, in order to generate the focus error signal, it was necessary to provide a cylindrical lens or a meniscus lens on the upstream side of an optical sensor, so that astigmatism of the reflected light is directed to an angle of 45° with respect to directions of division of the optical sensor.
Since the cylindrical lens or the meniscus lens are relatively expensive, and it was necessary to perform the positional adjustment with high degree of accuracy so that a light spot of the reflected light is focused on a light receiving surface of the optical sensor, there was such problem that the cost of the optical pickup device as a whole increase significantly as a result of using these lenses.
On the other hand, there is also a known system for controlling the direction of the astigmatism of the reflected light so as to corresponds to the directions of division of the optical sensor without using the cylindrical lens or the like by rotating the direction of the entire optical system of the optical pickup by 45° about an optical axis of the objective lens. However, in this system, the following problems arise.
In other words, although the laser light used in the optical pickup device is generally generated by a semiconductor laser, the shape of the light spot of the laser light of the semiconductor laser is oval. Therefore, in the configuration in which the entire optical system is rotated by 45°, a major axis of the light spot focused onto the optical disk is inclined by 45° with respect to a data track. Such a configuration does not cause any problem in optical disks which have no fear of generating crosstalk between adjacent bits on the data track, such as CDs or DVD-ROMs. However, in the case of optical disks which require a high resolution for detecting adjacent two bits on the data track, such as writable DVD or optical disks for blue-violet laser, it is not preferable because the resolution in this direction is lowered.
A technology for generating astigmatism in the predetermined direction by setting an inclined parallel plate instead of a cylindrical lens is disclosed in JP-A-2002-245660. However, only by inclining the parallel plate, an unnecessary aberration is occurred in a sub-beam when it is applied to a three-beam system optical pickup, and hence detection of the tracking error signal based on the differential push-pull method is adversely affected.